The leading region of IncIl plasmid ColIb-P9.

The IncIl conjugative plasmid ColIb-P9 carries a psiB gene that prevents induction of the SOS response in host bacteria. This locus was found to be located 2.5 kb downstream of the ssb (single-stranded DNA- binding protein) gene in the leading region of ColIb. This portion of the plasmid is transferred first to the recipient cell during conjugation and is strikingly similar to part of the leading region of the otherwise distinct F plasmid. Determination of the nucleotide sequence of ColIb psiB demonstrated that the gene has 84% identity with the psiB gene of F. Promoterless lacZ fusions on ColIb were created to leading region genes ssb and psiB and to sog, a representative transfer gene. It was found that expression of all three genes is increased when the ColIb transfer system is derepressed, but ssb and psiB are expressed at a much lower level than sog. Expression of psiB and ssb is increased when the host cell is exposed to UV- irradiation or mitomycin C treatments. The DNA-damage inducibility of ssb and psiB is recA and lexA-independent showing that neither gene is a component of the SOS regulon. Expression of both psiB and ssb is strongly enhanced in conjugatively infected recipient cells. No enhanced synthesis of Sog polypeptides was detected following conjugation showing that the zygotic induction of ssb and psiB is not a general property of plasmid genes. The implication is that PsiB and SSB proteins function in the transconjugant cell, rather than in the primary donor. It has been proposed that PsiB acts to prevent triggering of the SOS response during conjugation by transferring single-stranded DNA. Consistent with this hypothesis, carriage of the psiB gene by ColIb was shown to prevent a low level of SOS induction following conjugation. Plasmids that carry ssb and psiB genes have replicons belonging to the RepFIC family. It is postulated that the trigger for SOS induction during conjugation may be generated during the initial replication of the plasmid in the newly infected recipient cell rather than by the process of single-stranded DNA transfer